This application is based upon and claims the benefit of priority of Japanese Patent Applications No. 2001-344296 filed on Nov. 9, 2001 and No. 2001-348972 filed on Nov. 14, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a flow amount control device.
2. Description of the Prior Art
Conventionally, a common rail fuel injection system is known typically as a system for supplying fuel to a diesel engine (hereinafter called an engine). The common rail fuel injection system is provided with an accumulation chamber (common rail) commonly communicating with each of cylinders of the engine. A fuel injection pump, whose fuel discharge amount is variable in each stroke, supplies high pressure fuel by necessary amount to the common rail so that pressure of fuel accumulated in the common rail is kept constant.
To keep pressure of the fuel accumulated in the common rail constant, flow amount of fuel to be supplied to the fuel injection pump is adjusted according to conditions of engine load so that flow amount of fuel discharged from the fuel injection pump is controlled. In the conventional common rail fuel injection system, a flow amount control device is provided between the fuel injection pump and a low pressure pump for feeding fuel to the fuel injection pump so that the flow amount of fuel supplied to the fuel injection pump and the flow amount of fuel discharged from the fuel injection pump are controlled.
In the flow amount control device incorporated in a pump device, a column shaped valve element is slidably moved within a cylindrical valve body by electromagnetic force of a drive member. Movement of the valve element in the valve body causes to change a relative position between adjusting ports or grooves formed in the valve element and communication bores formed in the valve body. The flow amount of fuel to be supplied to the fuel injection pump is adjusted by changing opening areas of the communication bores actually opened to the adjusting ports or grooves.
In the flow amount control device, each shape of the communication bores or each shape of the adjusting ports or grooves has to be determined in consideration of operation characteristic of an electromagnetic drive member under conditions that, according to changes of current applied to the drive member, the flow amount of fuel discharged from the flow amount control device, that is, flow amount characteristic, has to be accurately changed with a given non-linear relationship. Accordingly, it is costly to form the valve element having such adjusting ports or grooves or the valve body having such communication bores.
Further, generally, the moving amount of the valve element corresponds to an opening degree of acceleration valve for driving an engine. When a change of the opening degree of the acceleration valve is relatively small, for example, at a constant speed cruising time, the moving amount and speed of the valve element is relatively small. In addition, when the engine is not operated for a long time, the valve element does not move and rests in place for a long time. In these cases, a sliding clearance between the valve element and the valve body is not sufficiently filled with film of oil (fuel) for lubrication. In particular, in use of fuel whose viscosity is lower and which has poorer lubrication characteristic, adequate formation of the film of oil for lubrication is rather difficult, which causes higher frictional resistance at sliding surfaces between the valve element and the valve body so that the accurate flow amount control of the device can not be achieved.
Furthermore, since the flow amount control device is installed on an inlet side of the fuel injection pump so that pressure of oil flowed into the flow amount control device is relatively low, the fuel is not forced to flow into the sliding clearance between the valve element and the valve body.
In a case that an axial length of a cylindrical outer circumference of the valve element that slides on a cylindrical inner surface of the valve body is relatively long, an axial middle part of the outer circumference of the valve element is not sufficiently filled with fuel for lubrication.
An object of the present invention is to provide a flow amount control device with less deviations of flow amount characteristic and at lower manufacturing cost.
It is an aspect of the present invention to provide the flow amount control device in which a valve element smoothly moves slidably in a valve body with less frictional resistance therebetween.
Another aspect of the present invention is to provide the flow amount control device having the valve element and valve body, whose constructions are simpler.
To achieve the above object, in a device for controlling flow amount of fluid flowing between first and second fluid passages, a valve body has a cylindrical inner circumferential wall within which the first fluid passage is formed, an outer circumferential wall outside which the second fluid passage is formed and at least a communication bore which penetrates from the inner circumferential wall to the outer circumferential wall. An inner opening of the communication bore on a side of the inner circumferential wall comes in communication with the first fluid passage and an outer opening of the communication bore on a side of the outer circumferential wall communicates with the second fluid passage. A column shaped valve element is slidably disposed inside the inner circumferential wall. A drive member causes the valve element to move reciprocatingly in an axial direction of the valve body so that an area of the inner opening opened to the first fluid passage is variable according to changes of position of the valve element relative to the valve body.
With the device mentioned above, at least one of the valve element and the valve body is provided within sliding surfaces between the valve element and the valve body with at least a ring shaped groove extending circumferentially.
Since fuel is stored in the ring shaped groove, the fuel serves to form film of oil for lubrication in a sliding clearance between the valve element and the valve body so that frictional resistance therebetween is smaller. Accordingly, the device can control flow amount of fuel accurately.
The ring shaped groove may be formed only on an outer circumference of the valve element, only on the inner circumferential wall of the valve body or both on the outer circumference of the valve element and on the inner circumferential wall of the valve body.
Further, the ring shaped groove may be a plurality of grooves axially spaced. The number of the grooves and axial intervals between the grooves can be determined adequately according to an axial length of the column shaped valve element or stroke of the valve element.
As an alternative, the valve element may be provided at an axial end thereof with a spherical surface whose outer circumference perpendicular to the inner circumferential wall is in annular line contact with the inner circumferential wall.
With this construction, fuel flowing to the communication bore from the first fluid passage is guided by the spherical surface of the valve element so that fuel smoothly flows from the first fluid passage to the communication bore without substantial turbulent flow. Accordingly, the manufacturing cost of the valve element is lower.
It is preferable that the valve element is a round ball. In this case, only the valve element is in annular line contact with the inner circumferential wall of the valve body so that frictional resistance between the valve element and the valve body is smaller. Accordingly, it is not necessary to form the ring shaped grooves for forming the film of oil for lubrication.
As another alternative, the valve element may be composed of a column shaped body and a hemispherical body which are integrated into one piece and outer circumference of the column shaped body, whose diameter is equal to that of the outer circumference of the spherical surface in annular line contact with the inner circumference, is in surface contact with the inner circumferential wall.
To reduce frictional resistance between the outer circumference of the column shaped body and the inner circumferential wall of the valve body, the column shaped body may be provided midway between opposite axial ends thereof with at least a ring shaped groove formed along the outer circumference thereof and/or the valve body may be provided within the inner circumferential wall on which the column shaped body slides with at least a ring shaped groove formed along the inner circumference thereof.